At a basic level, a GC system contains an injector, an analytical column, a column oven, and a detector. A sample of material to be analyzed enters the analytical column through the injector along with a carrier gas for carrying compounds to the detection device. The analytical column is positioned inside a temperature controlled column oven and separates the constituent compounds of the material sample for detection by the detector. The column oven can be held at a constant isothermal temperature, or can be programmed to increase temperature over time, to separate and elute the constituent compounds of the material sample. Once released from the column, the carrier gas sweeps the compounds to the detector. Each measured compound in the material sample is represented as a peak on an output chart. The retention time of each compound, as shown by the peaks, is used by the GC system to identify the chemical makeup of the material sample. An example of an isothermal column oven with a direct heating column assembly is described in, for example, U.S. Pat. No. 7,228,067. An example of temperature programmed oven for moving a temperature gradient along a analytical column is described in, for example, U.S. Pat. No. 3,146,616.
Previous column ovens use a fluid (e.g., water, oil, air, or other liquid or gas) for temperature control of the thermal environment surrounding the analytical column. See, e.g., John V. Hinshaw, “Gas Chromatography Ovens”, LCGC Asia Pacific, Vol. 18, No. 1, pp. 17-21. At least one disadvantage of a liquid fluid was keeping the fluid from ruining the analytical column when changing the column, or from leaks through loose fittings. Air bath ovens also suffer from extended lag times in reaching a set-point temperature and added cooling requirements such as the systems shown in, for example, U.S. Pat. Nos. 3,053,077 and 4,181,613.
Once an isothermal oven has equilibrated from initial start-up, maintaining temperature is relatively easy because the oven temperature never changes between analysis runs. However, only compounds with similar boiling points can be separated at a single temperature. For the analysis of a broader range of compounds a temperature program is generally used. When a temperature program is initiated, there is a slight lag time between the column temperature and the set-point temperature of the ramp. As the temperature increases the actual oven temperature lags behind the set-point and the column lags more. This temperature lag is greater at higher ramp rates, because it takes longer for the heat to transfer from the oven element, through the air fluid, to the column. The GC system must be capable of repeating the same time and temperature profile from run to run. Choosing narrower and shorter columns for fast GC greatly increases the demands on the column oven. Additionally, temperature programmed ovens require a cooldown and equilibration time between runs. Common practice is to let air bath ovens equilibrate for an extra 2-4 min after the set-point has been reached, to allow this residual heat to dissipate.
Additional advances in temperature control circuits and turbulent mixing of air over the past few decades have resulted in modern GC designs permitting precise GC oven temperature control, although they still suffer from the setbacks described above with respect to equilibration time, residual heat, the stability of the environmental air, temperature lag, and the requirement of precise consistency from run to run.
While gas chromatography is a powerful tool for separating complex mixtures into individual components for identification and quantification, the GC analysis generally requires long analysis times, typically in the 30-60 minute range. Advances in fast GC analyses initially involved faster temperature ramping and faster oven cooling between analyses, using higher power heating elements and more powerful cooling fans. See, e.g., U.S. Pat. Nos. 4,923,486, 5,028,243, 5,215,556, 5,808,178, 5,114,439, 6,427,522, 9,194,849, and WO Pub. No. 2015/0144117. These have proved successful in reducing the heating and cooling cycle times to some degree. However, providing faster analyses in a traditional fluid oven requires compromises. Speeding up the GC temperature program alone can result in reduced column separation efficiency and reduced column lifetime. These systems also do not work with varying column types and/or lengths and still require fluid for controlling temperature.
Each of these fluid oven attempts at reducing the GC analysis times has limitations. Most GC methods are intended to detect compounds at the lowest possible levels, which implies that one needs to introduce as much material sample as possible into the analytical column for separation and detection. Short, narrow bore columns are not suited for low level compound analyses. Faster heating and cooling helps with longer analytical columns, but the column still needs come to temperature equilibrium before the next material sample is injected for the results to be reproducible, adding a few extra minutes to each cycle.